In an interactive communication system, a plurality of users or subscribers of the system are located in a predetermined region and are provided with the capability of interacting with their television, personal computers, etc. More particularly, the users are connected via a cable plant in a tree-like structure to equipment in a Central Office. Various formats can be used to permit the plurality of users to share the resources of the cable plant and the Central Office equipment. One of such format is the use, for example, Frequency Time Division Multiple Access (FTDMA).
U.S. Pat No. 5,138,635 (Ballance), issued on Aug. 11, 1992, discloses a technique for clock synchronization in a communications network. The network comprises a central exchange comprising a master clock, and a plurality of remote subscriber stations that are connected to the central exchange and communicate via data cells with the central exchange. The subscriber stations are arranged to recover a master clock frequency from data received from the central exchange, and to transmit data to the central exchange at that master clock frequency. The central exchange comprises (a) a delay line arranged to receive incoming data cells, (b) means for sampling data from different taps of the delay line, and (c) means for determining from the sampled data which tap of the delay line carries data optimally in phase with the master clock and for selecting that data for outputting a data cell accordingly. Therefore, the central exchange uses a single clock, but instead of attempting to recover an appropriate clock for the incoming data, the central exchange uses a delay line to provide a progressive delay for the incoming data via the delay line, and then identifies the tap from the delay line which carries the incoming data having a phase appropriate to the master clock. In this manner, the central exchange handles data transmitted from subscriber stations with a phase that varies arbitrarily with respect to the system master clock.
U.S. Pat. No. 5,425,027 (Baran), issued on Jun. 13, 1995, discloses a wide area fiber and TV cable fast packet cell network. The fiber portion of the network bidirectionally transmits Asynchronous Transfer Mode (ATM) compliant cells over a digital optical fiber path from a head end unit to interconnect at subscriber interface units (SIU) for supporting two-way digital services with a coaxial feeder cable TV system, where the TV and digital signals are transmitted at different frequency bands. At the subscriber terminals, the TV and digital signals are filtered and separately processed. Each SIU sends and receives a UHF signal that is converted to and from a digital signal, respectively, for conveying the ATM cells. Each ATM cell contains a local address of the source and destination of that cell, and the SIU only accepts those cells addressed to it. A capacity assignment and polling arrangement from a Fiber Terminating Unit (FTU) at the head end is used. More particularly, the FTU initially measures the round trip transit time of signals sent to each SIU. Each SIU is assigned a predetermined number of cells for transmission purposes which is monitored during transmissions and dynamically changed as required for most efficient use of the transmission medium. Timing for control of the SIUs uses bit timing of a constant bit stream of the cells emanating from the FTU, where each SIU locks a local oscillator to this bit rate to control the output cells of the SIU. This same timing source also provides the frequency reference for the receive section of each SIU. Thus the FTU and SIUs are essentially locked together with known measured transit time offsets.
U.S. Pat. No. 5,428,645 (Dolev et al.), issued on Jun. 27, 1995, discloses a technique for synchronizing a local time maintained at a node within a network architecture with a reference time. A burst, including a series of "k" synchronizing messages, is sent from a master node to remote slave nodes, where each synchronizing message includes a reference time stamp based on a reference time provided to the master node by a remote time source. The slave nodes synchronize themselves by first estimating a local time according to the slave node's reckoning which corresponds with the reference time determined in relation to the time stamps contained within the received synchronization messages. More particularly, the technique determines that a first time according to a first time scale falls between second and third times according to a second time scale. The times are selected based on synchronizing messages sent in a burst from a repository of one of the time scales (i.e., the reference time) bearing reference time stamps according to a reference time scale. These synchronizing messages are received by a local time repository, such as a slave node, which associates local time stamps according to the other time scale (i.e., a local time scale). Because of the protocol of the synchronizing messages, temporal relationships are determined between the local time stamps and the reference time stamps within the messages of a burst. These first, second, and third times are identified at the slave node based only on the synchronizing messages received within a burst. After the first, second, and third times are identified, a time is selected in relation to the second and third times, preferably halfway between them, and a difference is determined between the selected time and the third time. Finally, the local time is updated to compensate for such difference and is allegedly accurate to within half of the distance between the second and third times.
A problem exists in a Time Division Multiple Access (TDMA) communication access network of how to synchronize all of the plurality of remote users arranged in a tree-type architecture that may communicate with central office equipment using channels which may have different protocols so that all of the users have a common reference and know which frequency (channel), frame length, and number of time slot(s) they are to transmit on while operating with sufficient robustness and obtaining a shared allocation of the available resources. Synchronization of the plurality of users should be accomplished by distributing a time reference which is independent of a data rate, a physical channel, and a channel coding that is used in order to operate with any of the channel parameters that are now available for use or may become available for use in the future.